Lecture note on Subphylum: Cephalochordata

Cephalochordates (Greek: Kephale - head, Chorale - cord) are a group of small fish-like animals showing all the fundamental chordate characters, namely notochord, dorsal tubular nerve cord, pharyngeal gill slits and postanal tail throughout life. The name of the phylum came from the fact that the notochord extends beyond the beginning of the nerve cord up to the tip ofthe snout. Cephalochordates are popularly known as lancelets (= little spears) because of their peculiar body shape. As there is no skull, the group is also called Acrania. They are exclusively marine and prefer sandy littoral zone.

General Characters

·         The cephalochordates exhibit the following general characters:

·         Body symmetrical or asymmetrical without a distinct head and paired appendages. However, tail and median fins (dorsal, caudal and ventral) are present.

·         The body is covered with a soft skin devoid of any exoskeletal structures.

·         The musculature consists of a series of segmentally arranged muscle blocks, called myotomes or myomeres.

·         Notochord extends the entire length of the body dorsally.

·         A true coelom is present. It is enterocoelous and is greatly reduced owing to the development of an ectoderm-lined spacious cavity, called atrium.

·         Digestive system is simple, straight and adapted for filter feeding.

·         Specialized digestive glands are absent except for a midgut diverticulum.

·         Mouth and anus are sub-terminal.

·         Pharynx is very large and perforated by large number of gill slits. It serves as a specialised feeding apparatus. The gills communicate to the outside indirectly through atrium.

·         Circulatory system closed type without a distinct heart. Blood is colourless for want of respiratory pigments.

·         Nervous system simple and built on vertebrate plan comprising central,peripheral and autonomous divisions. Distinct brain is absent and sense organs are simple.

·         Respiration occurs by diffusion through general body surface and gillslits.

·         Segmentally arranged protonephridia with solenocytes or flame cells are the excretory organs.

·         Sexes are separate. Gonads are segmentally arranged and without gonoducts. Fertilisation external and development indirect with a free swimming larval stage.